Of the technology available for achieving high data rate transmission over optical fibers, densely- spaced wavelength-division multiplexing (WDM) is a promising candidate. Optical fibers are key components in modem telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
A typical optical fiber communications system comprises a source of optical input signals, a length of optical fiber coupled to the source, and a receiver coupled to the fiber for receiving the signals. One or more amplifying devices are disposed along the fiber for amplifying the transmitted signal. Pump energy must be supplied to operate the amplifier. Contemplated optical fiber systems use digitally modulated optical signals at a wavelength of 1.55 micrometers and erbium-doped fiber amplifiers.
One problem limiting the capacity of such systems is that the erbium-doped fiber amplifier has a characteristic spectral dependence providing different gain for different wavelength channels. This spectral dependence poses a problem for multichannel WDM systems, because different gains for different channels leads to high bit error rates in some of the channels. In this case, a spectral shaping device helps flatten the gain spectrum of the amplifier.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA. which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA. which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.&gt;10.lambda.. Typically .LAMBDA. is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5.LAMBDA. to 4/5.LAMBDA.. In some applications, such as chirped gratings, the spacing .LAMBDA. can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the fiber core, long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode. The spacing .LAMBDA. of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA. can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped to provide a wavelength dependent loss.
Long-period grating devices are thus useful as filtering and spectral shaping devices in a variety of optical communications applications. Key applications include spectral shaping for high-power broadband light sources (C. W. Hodgson, et al., 9 Optical Society of America Technical Digest Series, Paper TuG3 (1996)), gain equalization for optical amplifiers (A. M. Vengsarkar et al. 21 Optical Letters 336, (1996)), band rejection in cascaded high-power Raman lasers (S. G. Grubb et al., Laser Focus World, p. 127 (February 1996)), and filtering amplitude spontaneous emission in erbium doped amplifiers (A. M. Vengsarkar et al., 14 J. Lightwave Technol. 58 (1996)). See also U.S. Pat. No. 5,430,817 on "Optical Systems and Devices Using Long Period Spectral Shaping Devices" by A. M. Vengsarkar, issued on Jul. 4, 1995.
One use of the long-period gratings is flattening the gain of broadband amplifiers for WDM systems. For WDM applications, multiple channels, each operating at several gigabits per second, must be accommodated within the 1530- to 1560-nm band of an erbium-doped fiber amplifier (EDFA). A nonuniform amplifier gain profile, however, leads to uneven signal amplitudes in the different channels, an effect exacerbated by the gain-peaking tendency of an amplifier chain. This amplitude variation between channels can be canceled out by a device with a transmission spectrum matched to the inverted erbium gain spectrum.
A difficulty with conventional long-period gratings, however, is that their capability to equalize amplifier gain is limited, because they filter only a fixed wavelength acting as wavelength-dependent loss elements. Each long-period grating with a given periodicity (.LAMBDA.) selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p =(n.sub.g -n.sub.ng).multidot..LAMBDA., where n.sub.g and n.sub.ng denote the effective index of the core mode and the cladding mode, respectively. The values of n.sub.g and n.sub.ng are dependent on the relative values of the refractive indices of the core, cladding, and air.
In a long-period grating, transmission loss can be tailored by inducing different levels of index change in the fiber during fabrication, i.e., by choosing different materials for fabricating the core and cladding of the optical fiber. For example, an erbium gain spectrum is first decomposed into a sum of two Gaussians. Two filters are fabricated, their shapes tailored by varying the exposure times. These gratings are then concatenated to produce a composite transmission spectrum. Used in conjunction with an erbium-doped fiber amplifier, this device can yield a flat gain spectrum with variation less than 0.2 dB over a 25- to 30-nm band.
While long-period gratings may be fabricated to address the transmission loss in WDM systems, the effectiveness of the gratings is yet limited. In the future, multi-wavelength communication systems will require reconfiguration and reallocation of wavelengths among the various nodes of a network depending on user requirements, e.g., with programmable add/drop elements. This reconfiguration will impact upon the gain of the optical amplifier. As the number of channels, passing through the amplifier changes, the inversion level of the erbium ions is altered, leading to non-uniform gain shapes. Thus, the amplifier will start showing deleterious peaks in its gain spectrum, requiring modification of the long-period grating used to flatten the amplifier. Modifying the long-period grating implies altering either the center wavelength of the transmission spectrum or the depth of the coupling.
Accordingly, there is a need for a long-period grating for use as a gain equalizer whose transmission spectrum can be controlled as a function of the number of channels and power levels transmitted through the amplifier. It is desirable to have a tunable (or reconfigurable) long-period grating which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p.) Further, it is desirable to be able to selectively filter a broad range of wavelengths, e.g., for efficient operation of multiple-channel WDM in telecommunication systems. This invention discloses such a tunable long-period grating device.